Diinosine pentaphosphate (IP5I) is a potent antagonist at recombinant rat P2X1 receptors

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Abstract
The antagonist activity of a series of diinosine polyphosphates (IpnI, where n=3, 4, 5) was assessed against ATP‐activated inward currents at rat P2X1–4 receptors expressed in Xenopus oocytes and studied under voltage‐clamp conditions. Diinosine polyphosphates were prepared by the enzymatic degradation of their corresponding diadenosine polyphosphates (e.g., Ap5A into Ip5I) using 5′‐adenylic deaminase, and purified using reverse‐phase chromatography. Against ATP‐responses at rP2X1 receptors, the potency order for antagonism was (pIC50): Ip5I (8.5)>Ip4I (6.3)>Ip3I (>4.5). Ip5I (10–100 nM) caused a concentration‐dependent rightwards displacement of the ATP concentration‐response curve without reducing the maximum ATP effect. However, the Schild plot was non‐linear which indicated Ip5I is not a competitive antagonist. Blockade by micromolar concentrations of Ip5I was not surmountable. Ip4I also behaved as a non‐surmountable antagonist. Against ATP‐responses at rP2X3 receptors, the potency order for antagonism was (pIC50): Ip4I (6.0)>Ip5I (5.6)>Ip3I (>4.5). Blockade by Ip4I (pA2, 6.75) and Ip5I (pA2, 6.27) was surmountable at micromolar concentrations. Diinosine polyphosphates failed to inhibit ATP‐responses at rP2X2 receptors, whereas agonist responses at rP2X4 were reversibly potentiated by Ip4I and Ip5I. None of the parent diadenosine polyphosphates behave as antagonists at rP2X1–4 receptors. Thus, Ip5I acted as a potent and relatively‐selective antagonist at the rP2X1 receptor. This dinucleotide pentaphosphate represents a high‐affinity antagonist for the P2X1 receptor, at which it acts in a competitive manner at low (100 nM) concentrations but has more complex actions at higher (>100 nM) concentrations. British Journal of Pharmacology (1999) 128, 981–988; doi:10.1038/sj.bjp.0702876